Partial discharges (PD) are often a precursor to failures of electrical insulation such as the oil impregnated paper dielectric used in power transformers. The PD is a result of an electrical stress which locally exceeds the withstand level of the dielectric (or insulation) resulting in an electrical avalanche; that is, a discharge. The discharge is local since it does not bridge the entire dielectric system causing an undesirable power arc but merely discharges a small volume of the dielectric system. Hence the designation "partial". If, however, the discharge is continuing, eventually it would be destructive to at least the dielectric system affected by the discharge or it could spread into a full destruction of the dielectric system of the transformer.
The general concept of the foregoing partial discharge phenomenon in large power transformers has been known but there has been no practical on-site electrical monitoring technique developed. In other words, at present electrical PD detection is limited to laboratory facilities used for high voltage testing of power transformers. In addition there has been the use of measurements made of the ultra-sounds generated by a discharge.